The present invention relates to a hot water heating system for blending cold water with overheated hot water to form a stream of hot water having a predetermined temperature. In particular, the present invention relates to a control valve assembly that maintains the stream of hot water at a nearly constant temperature over a wide range of flow rates of the hot water.
In many hot water heating systems such as institutional systems, cold water is heated by steam in a heat exchanger. For practical reasons, the output flow from the heat exchanger is overheated and is much too hot to be used at a hot water tap. Accordingly, the overheated water is blended with cold water in a blending chamber until a mixture having temperature suitable for the hot water tap is obtained.
Difficulties have been encountered in producing a control valve assembly that can maintain the temperature of the hot water substantially constant over a wide range of flow demand. To maintain the temperature of the hot water leaving the blending chamber substantially constant over the range of flow rates, the ratio between the overheated water arriving in the blending chamber and the cold water arriving in the blending chamber must be varied. There are two reasons for this. First, the pressure drop associated with the heat exchanger changes as the rate of flow through the heat exchanger changes. Second, the overheated water temperature tends to fall with increase of flow through the heat exchanger.
A substantial breakthrough in control valve assemblies for hot water heating systems described above has been achieved in a valve control assembly disclosed in U.S. Pat. No. 4,219,044 issued to the present inventor and incorporated herewith by reference.
In the control valve assembly disclosed in U.S. Pat. No. 4,219,044, the flow of cold water into the valve assembly is split. A portion of it flows through an inlet valve into the heat exchanger, and a portion of it flows through a by pass valve into the blending chamber. The ratio of the cold water flow into the heat exchanger and the cold water flow into the blending chamber is regulated by a controller. The controller comprises a diaphragm that actuates a valve stem connected with the inlet and bypass valves.
Two hot water temperature adjustments are provided to assure that the temperature of the hot water is very near constant over a wide range of demand. A first or low-flow temperature adjustment is made when the demand for blended hot water is small, about 10% of capacity. The amount of cold water admitted through the bypass valve into the blending chamber is adjusted by moving a bypass valve seat toward or away from the bypass valve member until the desired blended hot water temperature is achieved. The second or high flow temperature adjustment is made when the flow is about 50% of capacity.
While performing quite satisfactory at most flow rates, the control valve assembly of U.S. Pat. No. 4,219,044 experiences some difficulties in maintaining the desired blended water temperature at high flow rates. At high flow rates, the temperature of the overheated water that enters the blending chamber from the heat exchanger falls. This decrease in overheated water temperature results from increased flow rates through the heat exchanger. Accordingly, less cold water for blending is required at the high flow rates, while at lower flow rates an increasing amount of cold water for blending is required.
To provide for less cold water for blending at the high flow rates, a flow restrictor controls flow of the cold water into the blending chamber. The flow restrictor moves to restrict the flow of cold water into the blending chamber at the high flow rates so that less cold water enters the blending chamber.
According to the invention, the control valve assembly comprises an inlet valve and a bypass valve. A diaphragm controller which is actuated by the difference in the fluid pressure at the inlet and the outlet of the control valve assembly controls the positions of the inlet and bypass valves in accordance with the required flow rate of the hot water. A stem fixedly connected with the diaphragm of the diaphragm controller mounts the inlet and bypass valve. The inlet valve includes an inlet valve member and a valve seat with which the inlet valve member cooperates. The inlet valve member is fixedly connected to the stem. The bypass valve includes a bypass valve member which is also connected to the stem and a bypass valve seat. The restrictor is adjustably connected to the stem for joint movement therewith and relatively thereto. When the first, low-flow temperature adjustment is made, the amount of cold water admitted through the bypass valve into the blending chamber is adjusted by moving the bypass valve seat toward and away from the bypass valve member. The second or high-flow temperature is effected by moving the restrictor along the stem.